In hydrogenating organic material using zerovalent metal catalysts, it is more common to use the metal dispersed on an inert support than to use, for example, colloidal dispersions of the metal itself. Among advantages accruing to supported metals are included their greater surface activity, leading to increased reactivity, and their greater ease of separation, as by filtration. Colloidal metals are notoriously difficult to separate by filtration, and incomplete removal and recovery is costly and often deleterious to the product of hydrogenation.
When hydrogenations are conducted in aqueous media, the lack of hydrothermal stability of the commonly used supports places severe limitations on catalyst lifetime and recovery and also on the quality of the product due to dissolved support material. Where such hydrogenations are of hydroxylic organic compounds, the problem of hydrothermal instability of support materials is intensified. Where the organic compounds are polyhydroxylic, such as carbohydrates, the problem of hydrothermal instability is particularly exacerbated because of the relatively high concentration of hydroxyl groups from both water as solvent and the material to be hydrogenated.
The irony in hydrogenating aqueous solutions of carbohydrates is two-fold. First, the reduction products of many carbohydrates are important materials of commerce; sorbitol and mannitol are but two common reduction products. Additionally, there is no practical alternative to using water as the solvent in hydrogenating carbohydrates because carbohydrates generally are insoluble or, at best, sparingly soluble in most organic solvents. And because carbohydrates are solids, it is operationally mandatory to use a solvent in their hydrogenation.
Therefore, it is an object of this invention to hydrogenate carbohydrates in aqueous media using as a catalyst a zerovalent metal on a hydrothermally stable support. An embodiment comprises a method of hydrogenating an aqueous solution of a carbohydrate where the catalyst is a Group VIII zerovalent metal dispersed on a support of titanium dioxide, TiO.sub.2. In a more specific embodiment, the metal is nickel. In a still more specific embodiment, the nickel on titanium dioxide combination is reduced at a temperature less than about 300.degree. C. In a yet more specific embodiment, the carbohydrate is a hexose.